‘T’ be or not ‘T’ be...?
Triumph claim ‘feel’ and traction benefits from the unusual T-plane crankshaft in their Tiger triples. Jamie Turner explains
Of all the recent engine developments, Triumph’s adoption of its T-plane crankshaft configuration is one of the most unusual. Which therefore makes it one of the most interesting.
Triumph claim that the 180°-270°-270° intervals are to do with feel, with traction a part of it too. I guess for those with longish memories this will hark back to the Big-bang two-stroke engines of the 1990s (oh, the lost glory). The theory is that a tyre will slip ‘n’ grip more predictably if it has a large gap in its firing intervals during which to recover its composure. This is perhaps part of the reason why Ducati use what appears to be a really odd firing interval arrangement in their V4. Can’t think that moving from the 240° intervals of a normal triple to the T-plane order really amounts to a hill of beans as far as traction is concerned, though.
Those with really long memories are probably now shouting about the original Laverda triple which was, for an engine geek, really odd – it had a ‘flat-plane’ crankshaft, like an in-line four (except for Yamaha – more on that presently), with the centre crankpin pointing up while the outer pins were down. This meant that a heavier rod was needed in the middle pot to provide primary balance, replicating two cylinders’ worth of reciprocating mass in a normal I4. It also meant that it had a really odd set of firing intervals at 180°-180°-360°, so it sounded like a continually-misfiring four-cylinder.
The only reasons I can think why they did this is cost and to avoid using a balance shaft. The heavy rod can’t have helped bearing life, and it didn’t last long – subsequent Lav triples were conventionally-cranked. So, I’m not sure the T-plane owes anything to the Laverda, because (1) we’ve come a long way since then and (2) the Triumph has more than one plane anyway.
As well as the traction angle, there’s also the inertia torque thing. When Valentino Rossi went to Yamaha the M1 engine had classic even firing intervals. To paraphrase the late, great John Robinson: ‘Give an engineer a clean sheet of paper and they’ll design an engine with the best state of balance and as close to even firing as they can get. They just can’t help themselves.’ And indeed, that’s how we are programmed. But when you get near a vehicle, sometimes other things crawl out of the woodwork. Rossi reckoned he could feel the effect of all the pistons being at the dead centres together – giving the so-called inertia torque problem. To detect it you must have superhuman senses like Rossi I guess, but Yamaha went to the cross-plane crankshaft. It’s a good story but in reality it’s probably more a result of a heavier flywheel being necessary to reverse all of the pistons’ motions simultaneously. In this sense, because two pistons are now at their dead centres simultaneously, a T-plane triple is worse than a normal 120° crank. Perhaps that gives better feel in off-road conditions…
In relation to this, the long-memoried folks might also remember the Yamaha TRX of the late 1990s. This was the original 270° crankshaft parallel twin, which are ten a penny now. Reputedly Yamaha wanted a better sound (i.e. like that of the Ducati 900SS they were targeting), but it’s interesting to reflect that in such arrangements the two pistons do not
‘It harks back to the Big-bang two-stroke engines of the 1990s (oh, the lost glory)’
reach their dead centres together as they do in 180° or 360° crank twins. Perhaps this is where Yamaha got their inertia torque understanding from, having got their first with the T-rex (hey, this was the era of Jurassic Park). Nice thought. Finally, dealing with the changes in balance with the T-plane is interesting. A conventional even-firing triple has a first-order couple balanced with a counter-rotating shaft, which would just be reconfigured for the T-plane to deal with what is now a residual out-of-balance force. But the second order couple of ‘normal’ triples – which is usually left unbalanced – will become a small second order force acting in line with the cylinders. Forces are more noticeable and riders reinforce this. At least that bit can be explained. The fact that it is not – yet – in high-output engines cannot go unremarked upon, because uneven firing makes tuning more problematic in four-strokes.
So there you have it. We should take Triumph’s word for it all. And it’s useful Triumph starts with a ‘T’, too.